Re: [RFC PATCH 5/5] selftest/x86: Add CET quick test
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From: Kees Cook <hidden>
Date: 2020-05-21 23:02:55
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linux-arch, linux-doc, linux-mm, lkml
On Thu, May 21, 2020 at 02:17:20PM -0700, Yu-cheng Yu wrote:
Introduce a quick test to verify shadow stack and IBT are working.
Cool! :) I'd love to see either more of a commit log or more comments in the test code itself. I had to spend a bit of time trying to understand how the test was working. (i.e. using ucontext to "reset", using segv handler to catch some of them, etc.) I have not yet figured out why you need to send USR1/USR2 for two of them instead of direct calls? More notes below...
quoted hunk ↗ jump to hunk
Signed-off-by: Yu-cheng Yu <redacted> --- tools/testing/selftests/x86/Makefile | 2 +- tools/testing/selftests/x86/cet_quick_test.c | 128 +++++++++++++++++++ 2 files changed, 129 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/x86/cet_quick_test.cdiff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile index f1bf5ab87160..26e68272117a 100644 --- a/tools/testing/selftests/x86/Makefile +++ b/tools/testing/selftests/x86/Makefile@@ -14,7 +14,7 @@ CAN_BUILD_CET := $(shell ./check_cc.sh $(CC) trivial_program.c -fcf-protection) TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \ check_initial_reg_state sigreturn iopl ioperm \ protection_keys test_vdso test_vsyscall mov_ss_trap \ - syscall_arg_fault + syscall_arg_fault cet_quick_test TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \ test_FCMOV test_FCOMI test_FISTTP \ vdso_restorerdiff --git a/tools/testing/selftests/x86/cet_quick_test.c b/tools/testing/selftests/x86/cet_quick_test.c new file mode 100644 index 000000000000..e84bbbcfd26f --- /dev/null +++ b/tools/testing/selftests/x86/cet_quick_test.c@@ -0,0 +1,128 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* Quick tests to verify Shadow Stack and IBT are working */ + +#include <stdio.h> +#include <stdlib.h> +#include <signal.h> +#include <string.h> +#include <ucontext.h> + +ucontext_t ucp; +int result[4] = {-1, -1, -1, -1};
I think you likely want three states: no signal, failed, and okay. Perhaps -1 for "no signal" like you have above, zero for failed, and 1 for okay.
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+int test_id; + +void stack_hacked(unsigned long x) +{ + result[test_id] = -1;
So this is set to 0: "I absolutely bypassed the protection".
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+ test_id++; + setcontext(&ucp); +} + +#pragma GCC push_options +#pragma GCC optimize ("O0")
Can you avoid compiler-specific pragmas? (Or verify that Clang also behaves correctly here?) Maybe it's better to just build the entire file with -O0 in the Makefile?
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+void ibt_violation(void) +{ +#ifdef __i386__ + asm volatile("lea 1f, %eax"); + asm volatile("jmp *%eax"); +#else + asm volatile("lea 1f, %rax"); + asm volatile("jmp *%rax"); +#endif + asm volatile("1:"); + result[test_id] = -1;
Set to 0, and if the segv doesn't see it, we know for sure it failed.
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+ test_id++; + setcontext(&ucp); +} + +void shstk_violation(void) +{ +#ifdef __i386__ + unsigned long x = 0; + + ((unsigned long *)&x)[2] = (unsigned long)stack_hacked; +#else + unsigned long long x = 0; + + ((unsigned long long *)&x)[2] = (unsigned long)stack_hacked; +#endif +} +#pragma GCC pop_options + +void segv_handler(int signum, siginfo_t *si, void *uc) +{
Does anything in siginfo_t indicate which kind of failure you're detecting? It'd be nice to verify test_id matches the failure mode being tested.
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+ result[test_id] = 0; + test_id++; + setcontext(&ucp); +} + +void user1_handler(int signum, siginfo_t *si, void *uc) +{ + shstk_violation(); +} + +void user2_handler(int signum, siginfo_t *si, void *uc) +{ + ibt_violation(); +} + +int main(int argc, char *argv[]) +{ + struct sigaction sa; + int r; + + r = sigemptyset(&sa.sa_mask); + if (r) + return -1; + + sa.sa_flags = SA_SIGINFO; + + /* + * Control protection fault handler + */ + sa.sa_sigaction = segv_handler; + r = sigaction(SIGSEGV, &sa, NULL); + if (r) + return -1; + + /* + * Handler to test Shadow stack + */ + sa.sa_sigaction = user1_handler; + r = sigaction(SIGUSR1, &sa, NULL); + if (r) + return -1; + + /* + * Handler to test IBT + */ + sa.sa_sigaction = user2_handler; + r = sigaction(SIGUSR2, &sa, NULL); + if (r) + return -1; + + test_id = 0; + r = getcontext(&ucp); + if (r) + return -1; + + if (test_id == 0) + shstk_violation(); + else if (test_id == 1) + ibt_violation(); + else if (test_id == 2) + raise(SIGUSR1); + else if (test_id == 3) + raise(SIGUSR2); + + r = 0; + printf("[%s]\tShadow stack\n", result[0] ? "FAIL":"OK");
Then these are result[0] == -1 ? "untested" : (result[0] ? "OK" : "FAIL"))
+ r += result[0];
+ printf("[%s]\tIBT\n", result[1] ? "FAIL":"OK");
+ r += result[1];
+ printf("[%s]\tShadow stack in signal\n", result[2] ? "FAIL":"OK");
+ r += result[2];
+ printf("[%s]\tIBT in signal\n", result[3] ? "FAIL":"OK");
+ r += result[3];
+ return r;
+}
--
2.21.0-- Kees Cook